Paralytic poliomyelitis continues to be a major health problem in developing countries. Prevention strategies are based on polio vaccines which are either live, attenuated and orally administered (OPV) , or inactivated and parenterally administered. Globally, OPV has been favored for several reasons: it closely mimics alimentary and humoral immunity induced by wild type infection, production is simple and inexpensive, OPV is easy to administer, and may provide herd immunity. However, most immunogenicity data regarding OPV are derived from studies in developed countries, whereas among residents of developing countries, seroconversion to OPV has been poor. Evidence of this is demonstrated by recent outbreaks of poliomyelitis which have occurred in tropical areas despite high levels of immunization with OPV. These observations raise concern that routine OPV immunization schedules may be inadequate to achieve the goal of global eradication of wild poliovirus infection. Efforts to improve seroconversion in developing countries have therefore C)included the use of mass vaccination campaigns postulated to overwhelm individual and community enterovirus colonization. Data on the longterm effect of such campaigns on enterovirus circulation are limited, and the influence of infection with enteroviruses and other enteric viruses on OPV is still undetermined. The purpose of this study is to determine the effect of mass and routine OPV vaccination on the immunogenicity of OPV and to evaluate the relationship between infection with enteric viruses and the immunogenicity of OPV. The influence of mass versus routine vaccination, and the influence of enteric virus infection on the immunogenicity of OPV will be investigated by monitoring enteric infection with enterovirus, rotavirus, enteric adenovirus, astrovirus and Norwalk virus, and serologic response to OPV in newly vaccinated infants living in a rural Mexican village. Recently developed immunoassays and nucleic acid probes will be used to detect the study viruses.